For the gas turbine engine, in view of the environmental protection, strict criteria are applied, with respect to the composition of exhaust gases to be generated by combustion. In the criteria, reduction of harmful matters, such as nitrogen oxides (hereinafter, referred to as NOx), is greatly required. On the other hand, in the case of large-size gas turbines and/or engines for airplanes, from the requirements of reducing the fuel consumption and enhancing the output, the pressure ratio currently tends to be set higher. Associated with such a tendency, higher temperature and/or higher pressure operations are to be employed around the input port of the combustor. Therefore, due to such operations to elevate the temperature around the input port of the combustor, the combustion temperature may also tend to be higher, leading to further increase of NOx.
In recent years, a composite combustion method has been proposed, in which the lean pre-mixture combustion system that can effectively reduce the generation amount of NOx and the diffusion combustion system excellent in both of the ignition performance and the flame holding performance are combined together (see Patent Documents Nos. 1, 2, 3, 4, 5, 6 mentioned hereunder). In the lean pre-mixture combustion system, air and a fuel are mixed in advance so as to combust or burn the so-obtained mixed gas or mixture, with the fuel concentration of the gas being uniform. Thus, there should be no region where the flame temperature is locally elevated. In addition, the flame temperature can be lowered over the whole region due to the dilution of the fuel. Therefore, the amount of generation of NOx can be effectively reduced. However, because a great amount of air is mixed uniformly with the fuel, the local fuel concentration over the combustion region should be significantly low. Thus, the stability of combustion, especially under lower intensity combustion, may tend to be deteriorated. On the other hand, the diffusion combustion system is configured to perform combustion while diffusing and mixing the fuel and air. Therefore, flame failure of the combustion is not likely to occur even under lower intensity combustion, presenting a superior flame holding performance. Accordingly, the composite combustion system can ensure the stability of combustion, while starting the operation and/or operating under lower intensity combustion, due to its diffusion combustion region, while it can reduce the amount of generation of the NOx, under higher intensity combustion, due to its lean pre-mixture combustion region.
A combustor for the composite combustion system, as shown in FIG. 8, includes a fuel spray portion 81 adapted to spray a fuel so as to form the diffusion combustion region, due to the diffusion combustion system, in a combustion chamber 80, and a pre-mixture supply portion 82 shaped concentrically relative to the fuel spray portion 81 to surround the outer circumference of the fuel spray portion 81, and adapted for supplying a pre-mixture of a fuel and air so as to form the pre-mixture combustion region, due to the lean pre-mixture combustion system, in the combustion chamber 80. The combustor is configured to supply a fuel only from the fuel spray portion 81 while starting the operation and/or operating under a lower intensity combustion mode, whereas, on a higher intensity combustion mode, it supplies the fuel also from the pre-mixture supply portion 82, in addition to supplying of the fuel from the fuel spray portion 81. The fuel spray portion 81 includes a fuel atomizing portion 81a, which is adapted to change the fuel into particles suitable for combustion by utilizing shearing force of air, and a diffusion passage portion 81b disposed on the downstream side of the fuel atomizing portion 81a, adapted to diffuse the fuel and air at a speed suitable for the combustion, and having a spreading trumpet-like shape. In this case, it is intended to enhance the combustion efficiency, due to the diffusion combustion to be achieved by spreading the diffusion combustion region. Specifically, the spreading of the diffusion combustion region can be achieved by employing the diffusion passage portion 81b and a guide skirt member 81c further spreading out from the diffusion passage portion 81b up to the inner periphery of a downstream end of the pre-mixture supply portion 82.
Patent Document 1: JP No. 5-87340 A
Patent Document 2: JP No. 2002-115847 A
Patent Document 3: JP No. 2002-139221 A
Patent Document 4: JP No. 2002-168449 A
Patent Document 5: JP No. 2003-4232 A
Patent Document 6: U.S. Pat. No. 6,389,815
In such a configuration described above, while the fuel 84 is supplied only from the fuel spray portion 81 while starting the operation and/or operating under lower intensity combustion, only a great amount of air 85 is supplied into the combustion chamber 80 from the pre-mixture supply portion 82. As schematically shown in FIG. 8, the fuel 84 is injected in a direction toward the inner periphery of the downstream end of the pre-mixture supply portion 82 by air, along the diffusion passage portion 81b and guide skirt member 81c in a spreading trumpet-like shape. Thus, diffusion combustion flame 83 to be created by the mixture including the fuel 84 is also guided to spread into the entire space of the combustion chamber 80. However, the air 85 supplied from the pre-mixture supply portion 82 will interfere with the outer circumferential region of the so-created diffusion combustion flame 83. The interferential range between the diffusion combustion flame 83 and the air 85 is schematically depicted in FIG. 8, by using lattice-like hatching. The influence of such interference makes the local fuel concentration significantly lower at the outer circumferential portion of the diffusion combustion region as well as makes it difficult to keep the fuel concentration range to be suitable for stable combustion. Thus, flame failure in the diffusion combustion flame 83 may tend to occur, leading to deterioration of the ignition performance, the flame holding performance, and the stability of combustion under lower intensity combustion.
In particular, in the case of gas turbine engines used for airplanes, secure ignition is required under the conditions of lower temperature and lower pressure at a higher altitude, and various restrictions are imposed, with regard to harmful exhaust matters, such as CO and/or THC (Total HC), under lower intensity combustion, including idling time. Therefore, the degradation of the ignition performance and stability of combustion, due to the great amount of air 85 supplied from the pre-mixture supply portion 82 may often be problematic.